1. Field of the Invention
The field of the invention is industrial control systems including programmable controllers, and more particularly, communication links for connecting such controllers with remote input and output circuits.
2. Background Art
Industrial control systems such as those described in U.S Pat. Nos. 3,810,118, 3,942,158, 4,165,534 and 4,442,504 are typically connected to industrial equipment such as assembly lines or machine tools to operate such equipment in accordance with a stored program. The stored program includes instructions which, when executed, examine the condition of selected inputs to the system from sensing devices on the controlled equipment and energize or de-energize selected outputs from the system to operate devices on the controlled equipment.
Inputs to the system may be discrete binary signals such as those from switches which may detect limits of process variables such as motion, temperature, time, or other quantities, or the inputs may be analog measures of the process variables themselves, which are generally then converted to digital binary form for processing. Likewise, the outputs of the system may be either binary outputs as implemented by mechanical or solid-state relays, or analog outputs produced by means of a digital to analog converter.
The inputs and outputs processed by the system are received by the system through I/O modules. A number of I/O modules may be placed with a programmable controller in a rack containing a backplane which electrically interconnects the I/O modules with the controller. Alternatively, the I/O modules may be remotely located and connected to the controller by means of a communications "link". An I/O scanner incorporated into the controller allows the controller to send and receive data on this link.
Remote I/O modules may be grouped together within a remote rack having a backplane interconnecting the remote I/O modules. This remote rack is then connected to the serial link by means of an adapter. The adapter provides an interface between the data on the link and the backplane of the rack and provides a means for each I/O module to receive and transmit data on the link. Sixteen remote I/O racks may be connected in "parallel" on the link, that is, connected so that each rack may monitor the link simultaneously.
The link connecting the controller and the adapters of the remote I/O racks may take one of a number of topologies One topology is that of a multidrop or "daisy chain" configuration. In such a configuration, each adapter is positioned along the link so that it is connected directly with the controller and every other adapter. The adapters are each given a unique address and messages are directed to a particular adapter by transmitting the appropriate address before the message.
A remote rack may hold 16, 12, 8, or 4 I/O modules and therefore the address transmitted on the link before a message includes a rack address and a starting quarter to permit multiple racks with less than 16 I/O modules to share one rack address.
Orderly communication between the controller and the remote adapters is ensured by having all communications on the link initiated by the controller. If the controller needs to transmit or "write" data to the remote I/O racks, it transmits the data on the link and awaits the return of a confirmation signal. If the controller needs to receive or "read" data from the remote I/O rack, it transmits a data request on the link and awaits the return of the data from the remote I/O rack.
The address of each remote adapter is selected by the user and entered by switches on each adapter. This user-selected addressing permits maximum flexibility in configuring the industrial controller and its remote I/O. Remote I/O racks may be added or dropped as necessary and their corresponding addresses reassigned to minimize changes in the controller program.
This increased flexibility in configuring the industrial control system as provided by user-selectable addressing, carries with it the risk of two remote I/O racks being inadvertently assigned to the same address.
If duplicate addresses are assigned to two or more adapters, data communicated on the link may be corrupted by the interference of those adapters transmitting to the link simultaneously. Such interference may also result from multiple adapters responding to the same address as a result of adapter circuitry failure. Such multiple simultaneous transmissions are termed "collisions".
Industrial control systems are sensitive to even a low probability of collision even if the collisions may be reliably detected. The link between the controller and the remote I/O operates under severe real time constraints, and periodic retransmission of garbled data caused by a collision will unacceptably degrade the link's maximum response time. Also, the possibility an undetected collision and thus of undetected garbled data is a serious problem in the industrial environment where expensive and complex machinery is being controlled.
In some instances, a collision may cause electrical damage to high powered output circuits on the adapters when each attempts to take control of the link.